1. Field of the Invention
The present invention relates generally to a high-frequency switching transistor and particularly to a high-frequency transistor, which is known for a lower gating voltage than high-frequency transistors according to the prior art.
2. Description of Prior Art
It is the object of high-frequency switches to forward or block high-frequency signals. In the forward case, they should be characterized by an ohmic resistance, which is as small as possible, and in the blocking case, by a constant capacitance, which is as small as possible. They can be realized by different types of circuit elements. In the integrated circuit technology, high-frequency switches are often realized by using a silicon substrate. Thereby, generally, the two types of bipolar or MOS transistor can be formed. In the case of a silicon bipolar transistor, the forward case can be easily realized, if it is operated with a sufficiently large emitter base current in the triode area of the characteristic curve, far below the collector current saturation. At a correct transistor dimensioning, this control current can remain smaller than 1 mA. The blocking case causes more difficulties, since a base and substrate bias voltage, respectively, is required in the off-state, which is as high as the high-frequency amplitude to be blocked. In other words, it can be noted that a base emitter bias voltage has to be applied in the blocking direction with an amount of at least the amplitude of the high-frequency signal, so that the collector base diode is not controlled in the flow direction. Thereby, the switchable power is limited by the available bias voltage.
In battery-operated mobile radio systems, for example, this leads to the problem that a required bias voltage of about 20 volt is much higher than an operating voltage of the mobile radio system of, for example, 2.8 volt.
High-frequency switches for higher powers are realized outside the RF-ICs (RF-IC=radio frequency integrated circuits) in the form of PIN diodes or GaAs transistor switches, which can be operated with lower and no bias voltage, respectively. A disadvantage, however, is the higher cost which can occur due to the additional components.
Another disadvantage of PIN diodes as high-frequency switches is that PIN diodes require a partly higher switching time, which makes their usage in the high and highest frequency technique, respectively, problematic.